The present invention is directed to a method and an apparatus for characterizing a property of cardiac tissue, particularly, the ischemic state of cardiac tissue, from local electrograms.
Patients with ischemia frequently are asymptomatic at rest but experience symptoms under stress. A number of techniques currently exist for diagnosing ischemia in cardiac tissue. One such technique is stress electrocardiography (stress EKG), in which the electrocardiogram is measured while the patient is exercising. Other techniques for detecting ischemia include echocardiography and nuclear imaging. Both of these techniques may also be conducted while the patient is under stress, which may be either induced physically or pharmacologically with agents such as dubotamine. A technique for diagnosing chronic ischemia in patients purely from local electrograms taken when the patient is at rest has not heretofore existed. Furthermore, while some of the above-mentioned techniques can be used to diagnose ischemia at the regional level, they do not reveal ischemia with pinpoint accuracy.
U.S. Pat. No. 5,967,995 discloses a method for predicting life-threatening cardiac arrhythmias by gathering electrocardiographic data, mathematically decomposing the signal into several elements or components that contain the most significant information and tracking the changes in the several elements. The method of the ""995 patent does not permit evaluation and diagnosis of a patient without time-dependent historical data. Furthermore the ""995 patent does not teach or suggest the possibility of diagnosing the local ischemic state of tissue from local electrograms.
One aspect of the invention is directed to a method of characterizing a property of cardiac tissue at a local site of a heart based on a local electrogram measured at that local site. The method first involves normalizing the local electrogram. A feature vector is then extracted from the normalized electrogram. The property of the cardiac tissue at the local site is classified based on the resultant feature vector.
Another embodiment of the method of the invention is directed to characterizing a property of cardiac tissue at a plurality of local sites of a heart based on a plurality of local electrograms. This embodiment of the invention first involves normalizing the local electrograms. Feature vectors are then extracted from the normalized local electrograms. The property of the cardiac tissue at the plurality of local sites is then classified based on the extracted feature vectors
Another embodiment of the method of the invention is directed to characterizing a property of a region of cardiac tissue of a heart based on a plurality of local electrograms measured in the region. This embodiment first involves normalizing the electrograms. An electrogram which is representative of the tissue in the region is then computed from the normalized electrograms. A representative feature vector is then extracted from the representative electrogram. The property of the cardiac tissue in the region is then classified based on the representative feature vector.
Normalizing the local electrograms includes annotating the electrograms, selecting a single cardiac cycle from the electrogram, and representing the cardiac cycle of the electrogram as a vector such that the annotation is at a fixed position within the vector.
In some embodiments, the normalizing step further includes the step of scaling the electrogram. In this case, the electrogram is scaled so as to have a fixed value at a particular feature of the electrogram. The particular feature according to some embodiments corresponds to the annotation.
In some embodiments, the normalizing step further includes the step of centering the electrogram. The normalization step may also include rejecting outlying electrograms.
In some embodiments of the invention, the feature vector is a projection of the normalized electrogram onto a pre-computed subspace. The subspace may be computed by principal component analysis of a training set of electrograms. In this embodiment, the classifying step of the method of the invention is based on the location of the feature vector in the subspace.
In some embodiments, the property being characterized by the method of the invention is indicative of the anatomy of the local site or region. In other embodiments, the property is indicative of a pathological state of the cardiac tissue such as the degree of ischemia of the tissue at the local site or region.
Where the method of the invention involves characterizing the cardiac tissue at a plurality of sites or regions within a chamber of the heart, the method may further include the step of constructing a map of the property of the heart chamber.
In some embodiments, the method of the invention further includes the step of delivering treatment to the tissue at the local site or region. In such cases, the method may further include follow-up characterization of the tissue property to determine the effectiveness of the treatment.
In some embodiments, the local electrograms are measured with an electrode on a catheter. The catheter further comprises a position sensor such as an electromagnetic sensor for measuring the three-dimensional position of the electrode during measurement of the electrogram.
Another aspect of the invention is directed to apparatus for characterizing a property of cardiac tissue at a local site of a heart based on a local electrogram measured at the site. The apparatus of the invention includes a catheter and a processor. The processor performs the functions of normalizing the electrogram, extracting a feature vector from the normalized electrogram, and classifying the property of the cardiac tissue at the local site based on the feature vector. The processor may further perform the function of computing a map of the property of the heart tissue.
In some embodiments of the apparatus of the invention, the catheter, which includes an electrode for measuring the local electrogram, further includes a position sensor such as an electromagnetic sensor for measuring the three-dimensional position of the electrode during measurement of the electrogram.
The apparatus of the invention may further include means for delivering treatment to the tissue.
The present invention will be more fully understood from the following detailed description of preferred embodiments, taken together with the following drawings in which: